This commit adds support for listing arena players on arena signs by
introducing the following _dynamic variables_:
- `<arena-n>` the _nth_ live arena player
- `<lobby-n>` the _nth_ player in the lobby
- `<ready-n>` the _nth_ ready player in the lobby
- `<notready-n>` the _nth_ player in the lobby who hasn't readied up
Each variable points to a list of players (sorted by name), and the `n`
is the index into that list (1-indexed). This means that putting the
variable `<notready-1>` in a template will result in the name of the
first player in the list of lobby players who haven't readied up, sorted
by player name.
Implements #592
